Nega-Timmy is the evil alter ego of Timmy. He only appeared in the episode "Nega-Timmy".
Nega-Timmy is the result of Timmy being told by his dad to be good after he wished to do opposite of whatever his parents told him. His evil stare is enough to cause even Mr. Crocker to break down.
Nega-Timmy's appearance is the opposite of Timmy. He has two fangs, red eyes, tan skin, black hair, wears a cloak, is extremely intelligent, and is evil.
When Timmy wished he could do the opposite of what his parents tell him to do, it goes well until Mr. Turner tells him to be good. Timmy then turns evil and after a day at school, he tries to create a laser that would knock a satellite out of the sky which will cause it to fall and crash onto Mt. Dimmsdale with the lava flow resulting in the destruction of the Dimmsdale Dam (as that is the only natural disaster that could destroy it) creating a massive flooding disaster that will destroy the makeup factory and rid all beauty in Dimmsdale (as some people look terrible without makeup as stated by Wanda). His parents eventually found out about this, but Nega-Timmy enticed his father with the prospect of using the laser cannon to destroy Dinkleberg, at which point Mr. Turner told him to be as evil as possible so he could get his vengeance as much as he wished. This action turned Nega-Timmy back into Timmy just in time. Nega-Timmy has not appeared since then, possibly to avoid Timmy becoming evil again.